In recent years, variable geometry turbochargers (VGT) have been used in which turbines and compressors are downsized to allow an engine to be efficiently supercharged even at low engine speeds at which exhaust energy from the engine is low, and which include a turbine around which a plurality of movable flaps (referred to also as “movable vanes” or “nozzle vanes”) are arranged.
A variable geometry turbocharger is not usually provided with a wastegate through which exhaust gas is released upstream of the turbocharger. Thus, if the rotational speed of its turbine exceeds a predetermined threshold, the supply of fuel is controlled to reduce an increase in the rotational speed of the turbine.
Patent Document 1 teaches estimating the rotational speed of a turbine based on the amount of intake air and the boost pressure, and reducing the amount of fuel supplied if the overspeed of the turbine is anticipated.